Suitability of old apple varieties in organic farming, based on their resistance against apple scab and powdery mildew

Abstract

Apple scab and powdery mildew are the two most destructive fungal diseases of apple, especially in organic orchards, where synthetic pesticides are prohibited. Only a limited choice of natural substances for control of pests and diseases allowed to be used. Growing resistant/tolerant cultivars can highly reduce the number of the necessary plant protection treatments resulting in lower costs and environmental impact. Besides the newly developed cultivars, several old apple cultivars are thought to be resistant/tolerant to the main fungal diseases of apple; however, in most cases, the degree and type of their resistance is not well characterized. To evaluate the resistance of ten old cultivars against apple scab and powdery mildew, field evaluations were carried out for 6 years. Molecular marker analysis was also carried out: six different markers were used for the detection of three major scab resistance genes (Vf, Vh4, Vh2). The resistance gene Vf could not be found in any old cultivar while the resistance genes Vh2, Vh4 are possibly present in several cultivars. The results are explaining the good disease tolerance of several cultivars (e.g., ‘Batul’), and also suggest their relevance in the breeding as well as in organic farming. Some old cultivars showed good field resistance against fungal diseases, even though their resistance against scab could not be explained with the presence of the investigated major scab resistance genes. This suggests that a few cultivars might possess polygenic resistance, or maybe new major scab resistance genes.